Revising the Classical ‘Habitable Zone’

byPaul GilsteronAugust 3, 2018

With my time-out period over (more about this next week), I want to get back into gear with the help of Ramses Ramirez, a specialist on planetary habitability whose work has now taken him to Japan. Born and raised in New York City, Ramses tells me he is much at home in his new position as a research scientist at the Earth-Life Science Institute (ELSI) in Tokyo, where opportunities for scientific collaboration abound and the chance to learn a new language beckons. We’ve looked at Ramses’ papers a number of times in these pages, and I was delighted when he offered this description of his work to our readership. A student of James Kasting, he received his Ph.D. from Penn State in 2014 and went on to postdoc work with Lisa Kaltenegger at Cornell’s Carl Sagan Institute. A fascination with astrobiology and the issues involved in defining habitable zones continues to be a primary focus. Ramses’ new paper ponders whether we are best served by looking for life similar to Earth’s because this is what we know, or whether there is a broader strategy, one that probes all the assumptions in our ideas of the classical habitable zone.

By Ramses Ramirez

I am glad to have the opportunity to formally introduce myself and give a summary of the recent work on planetary habitability and the habitable zone. I define the habitable zone (HZ) as the circular region around a star(s) where standing bodies of liquid water could exist on the surface of a rocky planet [1]. The inclusion of the phrase “standing bodies of water” excludes dry worlds that may exhibit small outpourings of seasonal surface water (e.g. possibly Mars). Defined this way, the HZ is properly focused for detecting worlds that have large surface bodies of water (e.g. seas, big lakes, oceans) that are in direct contact with the atmosphere. If life is present on such a world, potential atmospheric biosignatures could be detected with current technology. However, in the absence of such large water bodies, life would not be detectable even if it were present. Likewise, although life may be possible within the seas of a Europa or Enceladus exoplanetary analogue, the global ice layer covering their oceans would prevent the detection of such subsurface life. Such observational issues keep the HZ within an orbital region that is placed somewhat closer to the star.

When I first started my Ph.D. in 2010 under my mentor Professor James F. Kasting, I did not know the exact topic that I would be working on at first, but with my interest in the search for extraterrestrial life, I knew that I wanted to work on planetary habitability. Fortunately, I got involved as one of the two lead authors on the 2013 HZ paper [2], where we updated the seminal Kasting et al. [3] HZ limits [2,3]. Although I was fascinated by the HZ concept as a navigational tool to find potentially habitable planets, my investigations soon led me to realize that the HZ definition described in these earlier works, what I dub the “classical HZ”, would be insufficient for capturing the diversity of such planets. From that point on, it became my personal mission to turn the HZ into an even more capable navigational tool.

The classical HZ assumes that CO2 and H2O are the key greenhouse gases on potentially habitable planets, following the carbonate-silicate cycle on the Earth, which is thought to regulate CO2 between the atmosphere, surface, and the interior (e.g. [3]). Given that the concept of a universal carbonate-silicate cycle on habitable exoplanets is itself far from proven (e.g., [4]), I always thought that this assumption was needlessly restrictive and geocentric. After all, HZ planets with atmospheres consisting of different gas mixtures can also support standing bodies of liquid water and potentially be habitable. Reduced greenhouse gases, like H2 and CH4, have been considered as major atmospheric constituents on both early Earth and early Mars (e.g., [5,6]). It has even been suggested that planets with primordial hydrogen envelopes may be habitable in some circumstances [7]. If hypothetical planets consisting of dense 10-bar CO2 atmospheres near the outer edge of the classical HZ are potentially habitable (e.g. [2][3]), then why not worlds composed of these other atmospheric constituents (Figure 1)?

Moreover, the classical HZ really targets potentially habitable planets orbiting main-sequence stars. However, this approach ignores the importance of the temporal evolution of the HZ, particularly a star’s pre-main-sequence phase. It is during this early stage that many M-stars are bright and luminous enough to completely desiccate any planets that are now thought to be located within the main-sequence HZ (e.g. [8][9][10]), like Proxima Centauri-b and many of the TRAPPIST-1 planets, unless such worlds are able to accrete enough water to offset losses (e.g., [11][12]) (Figure 2).

Figure 2: The pre-main-sequence HZ for a late (M8) star. A planet that forms at ~0.03 AU undergoes a runaway greenhouse state for ~100 Myr before finally entering the HZ, settling near the outer edge after ~1 billion years (reproduced from Ramirez [1]).

In my view, we are currently ill-equipped to infer the atmospheric conditions that are most suitable for extraterrestrial life. All we know is that life did somehow arise on this planet. It is therefore pretentious to take our one poorly-understood example and assume that alien life must follow a similar trajectory. Some might argue that we should focus on finding life similar to Earth’s because it is familiar and would be simpler to find. However, this argument from the principle of mediocrity is not convincing to me because we have no idea how common or rare our particular brand of life is until we are able to find a second instance of life’s occurrence elsewhere.

It is then illogical to rely on any one iteration of the HZ to find potentially habitable planets. Our inadequate understanding of biology, the origin of life, and planetary processes require that all of these working hypotheses, including the assumptions used in the classical HZ, be tested by observations. Ideas that are later found to be unsupported by nature can then be rejected whereas those that are confirmed should be refined. Only then will we begin to truly understand life’s possibilities and make more informed – and possibly more restrictive – design decisions for subsequent missions. But only proper observations (and possibly better theoretical constraints) can tell us this, and not adherence to some ambiguous and untested geocentric philosophy. Finding extraterrestrial life will be one of the hardest endeavors undertaken by our species. To meet the challenge, we need to start with a thorough and openminded search for potentially habitable planets.

Thus, as some Centauri Dreams readers may already know, part of my work (in collaboration with Lisa Kaltenegger) in recent years has focused on improving our understanding of both the spatial [13][14] and temporal evolution of the HZ [8][15]. These papers have already been discussed in previous Centauri Dreams posts. However, in addition to our own work, other researchers and colleagues have also proposed their own revisions and extensions to different aspects of the HZ, further increasing its utility as a tool for finding life that is both similar and dissimilar to Earth’s. This gradual but steady evolution in how we think about the HZ has come to a good point (I believe) for a review paper to come along such as this one.

I hope that your readers will like my new summary of recent advances (linked below) in our understanding of the HZ. I see this work as both a primer for the student and layman as well as a guide for space missions. It includes many recommendations and suggestions for how the classical HZ, in conjunction with newer HZ formulations, can be used to maximize our chances of finding extraterrestrial life. For example, I do not think that a proper assessment of planetary habitability can be made without properly assessing a planet’s pre-main-sequence habitability. A-stars should also be considered as potential hosts for life-bearing planets. Most importantly, we should employ the various HZ formulations and rubrics to rank which planets are most likely to host life. Finally, my paper clarifies many common misconceptions about the HZ concept and explains why the search must necessarily be limited to finding surface liquid water (at least for now).

The review paper is “A more comprehensive habitable zone for finding life on other planets,” published in Geosciences 8(8), 280 (28 July 2018). Full text available here. [PG: Expect a close look at this paper in coming weeks on Centauri Dreams].

Paul, Ramses, and Andrew LePage: The classical terrestrial/Neptunian boundary may ALSO have to be revised. The revised mass,radius, and density of LHS 1140b is as follows: 6.98 Mearth, 1.727 Rearth, and 7.5Gcm3. The EXTREME CHANGE in the radius SHOCKED ME! Do any of you know if this is due to a revised radius of the parent star as a result of GAIA DR2, or whether a small(Proxima Centauri to TRAPPIST-1)binary companion has been. If it is the FORMER, it would also move LHS 1140b from the outer part of the habitable zone(concervative) to the inner part of the habitable zone(optimistic). ALSO: Dr Ramirez; a month ago, the RE-revised masses of all of the TRAPPIST-1 planets were released with little to no fanfare, NOT in a paper, but at a meeting on exoplanets at Cambridge University. They are more in line with your predictions due to your concerns with the chi squared fits. Do you know whether this is a result of BETTER chi squared fits or GAIA DR2. RSVP.

Thanks for the updates on TRAPPIST-1 Harry. I did not attend that meeting so I am just now learning about the revised density/mass TRAPPIST-1 estimates. I do not know why the estimates have changed. Presumably, there will soon be a paper about this.

There is a new paper on TRAPPIST-1. “Non-detection of Contamination by Stellar Activity in the Spitzer Transit Light Curves of TRAPPIST-1.”, by Brett M Morris, Eric Algol, Leslie Hebb, Suzanne L Hawley, Michael Gillon, Elsa Ducrot, Laetitia Delrez, James Ingalls, Brice-Oliver Demory. According to the authors, there are NO large or medium starspots VISIBLE on TRAPPIST-1 AT ALL! Very VERY wierd!!! One can infer from this that the extreme flaring activity of TRAPPIST-1 is due to EITHER: Combined activity of EXTREMELY SMALL STARSPOTS below the detection level threshold of SST OR: Starspots OBSCURED by sone kind of “cloud deck” floating ABOVE the starspots.

“…there are no large or medium sized starspots AT ALL..” This means NEITHER dark NOR bright(as some astronomers have claimed to possibly detect). ALSO: Can’t rule out some phenomena OTHER THAN STARSPOTS such as magnetic re-connection due to to the interaction of the star’s and planets’ magnetic fields to be the root cause of the flares.

The change in LHS 1140 b radius is due to Gaia DR2, but it is insignificant to the planet’s position in the habitable zone for the planet was already close to the outer boundary. The revision to large stellar radius only moves LHS 1140 b to the middle of the conservative habitable zone.

> The classical terrestrial/Neptunian boundary may ALSO have to be revised.

While our understanding of the nature of the mass-radius relationship in the transition zone from rocky to volatile-rich planets is constantly evolving was more dataare accumulated, the latest revelations about the properties of LHS 1140b does not require a major revision of this relationship. The ~1.6 Re figure I and others have quoted does not represent a hard boundary with 100% of the planets smaller than this are rocky and 100% above this threshold are volatile-rich. We already know that the transition in the properties of the population of exoplanets is a gradual one with the proportion of volatile-rich exoplanets increasing more or less steadily starting somewhere around 1.2 Re. The ~1.6 Re figure represents a point where the population of exoplanets have a 50-50 split between rocky and volatile-rich worlds. Still larger rocky exoplanets are still possible (as is the case with LHS 1140 b with 1.7 Re) but it becomes increasingly unlikely with increasing radius (but not impossible).

Andrew: Thanks! I have been waiting for this! NOW: Tommi59 ALSO gave devised DENSITIES!(CAVEAT: These are HIS density estimates and NOT from PROFESSIONALS at the Cambridge University meeting): b-4.81g/cm3, c-5.23g/cm3, d-4.23g/cm3, e-4.80g/cm3, f-4.66g/cm3, g-4.61g/cm3, and h-4.39g/cm3. He states that if ALL of the planets had iron cores, substantial water would be needed to explain the densities of all of the planets with tjhe notable exception of TRAPPIST-1d. However, if the cores were made of LIGHTER MATERIAL, no water would be needed for ANY of them. Perhaps an UPDATED planet habitability reality check would be in order for BOTH TRAPPIST-1 AND LHS 1140. ALSO: More eclipses of more planets may be visible from the surfaces of planets OTHER than TRAPPIST-1e. Have fun computing them!

Paul, Ramses, and Andrew: Just in case ANY of you are NOT aware of the RE-revised masses, go to http://www.solar-flux.forumandco.com, click on “…latest discoveries”, scroll down to tommi59, click on “posts”, and scroll down six posts and you will find the chart with the RE-revised masses. Ironically, these masses are MUCH CLOSER to the ORIGINAL mass estimates of Gillon et al than either Grimm et al’s revised masses or those derived by the K2 observations.

Better yet, here they are: b-1.22 Mearth, c-1.24 Mearth, d-0.37Mearth, e-0.66Mearth, f-0.97Mearth, g-1.27 Mearth, h-0.37 Mearth. BUT: A POSSIBLE Mars-sized planet is NOW in the data, but ONLY ONE transit detected prohibits confirmation at this time. If it DOES exist, it is MUCH FARTHER OUT than the inner 7, which means there MAY BE SEVERAL NON-TRANSITING PLANETS IN BETWEEN IT AND THE INNER 7 that may be effecting the TTV data ON A CONTINUAL BASIS, resulting in DRASTIC CHANGES in the APPARENT masses of the inner seven planets over short periods of time.

Nope, Grimm et al and new results are systematically similar, and they marginally overlap with errors. The masses in Gillon et al have too great of uncertainties that cannot be compared with the new results and Grimm et al, though overlap.

Thank you for your very nice and interesting contribution Mr Ramses Ramirez. A nice link to the paper “Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs.” that I recommend to anyone on this forum.
The intense early stage of a M dwarf star will indeed break up a huge part of any available water, and might leave an oxygen atmosphere perhaps together with nitrogen that might remain for a long time.
Especially if the world get cold later so that the landmass get covered by ice, and ocean in other areas – so that the bedrock will not get very oxidised.
This is one type of world that could give a false positive, there are also others.
We do of course not know what alien life might be like, agreed, yet with an example of one, it would be funny if it turn out that silicone based life is the more common in the end – and we’re the rare exception.

My guess is that if really exotic biochemistries can exist, we will discover that in the lab. I’m not suggesting that we could develop silicone-based life de nono, but rather that we can show that silicones can be made into functional protein and DNA/RNA analogs under certain conditions, probably suited for industrial use.

Priors are not needed for all probabilities. The Drake equation, for example, has some fairly known probabilities, and some completely unknown ones, By plugging in ranges of probabilities as variables, you can still make [wide] ranges of output probabilities of events happening. In cases where the most sensitive variable probabilities are known, even fairly extreme ranges for the unknown probabilities can leave a relatively narrow range of outcomes. Those ranges are still useful as constraints.

Possibly the appearance and continuation of life is less dependent on the conditions we know than on the existence of equilibrium in a relatively dynamic situation. Maybe life becomes part of the balance as it evolves, depending first on stability.

Something else. Pure physics. There has to be some instability to provide energy on the ground and for evolution to even need to happen — making most life likely to be microscopic if a majority of planets are frozen or sun baked with no geologic activity to speak of. Complex organic activity will affect the environment because of physics, like it has here, not because of any mystical woo-woo.

DCM, Lovelock’s Gaia hypothesis was never woo-woo, even if idiots took it that way. Updated “Daisy World” models show that ecosystems can co-evolve with the planetary climate. Some transitions are more dramatic than others of course – the Great Oxidation Event, for example. Physics and biology influence each other given sufficient scale.

It is very laudable that we consider more carefully what may constitute an HZ world to help with our search for life.

Whether extended or not, the HZ concept is intrinsically tied to the idea of life evolving on an Earth-like planet and being found there alone. Any proposed search for life using remote methods relies on proxies to infer life may exist. We may get lucky and find a world and directly detect surface vegetation, but many planets may have life still in the prokaryotic state and perhaps even pre-photosynthetic. Confirming the presence or absence of life may require interstellar probes to sample the biospheres directly. That is a prospect centuries in the future with any conceivable non-FTL propulsion.

Conversely, in our own solar system, we could sample all worlds and moons directly. Should any of the more exotic ideas currently being followed by NASA prove correct, most notably life in icy moons, then the HZ concept is thrown almost into irrelevance concerning the presence of life. However we cannot detect life in these moons remotely, so they become, at best, a proxy for life around other stars.

Panspermia can also obviate the temporal constraints of HZs. Should life require a start on a suitable HZ world, but migrate, then the need for a single world to stay in an HZ is unnecessary and the model is extended again.

My guess is that since the “glory” will go to the first person to unambiguously discover life around another star, the focus will stay on searching for life on the most Earth-like worlds around medium temperature stars, as this is the most likely place to find terrestrial, multi-cellular, photosynthetic organisms whose presence is most unambiguous to remote observation.

In the far future, more sophisticated models of the type you are developing will no doubt be used to target interstellar probes and even crewed starships. One irony may be that those models will be used to avoid those living worlds based on some “Prime Directive”.

Thanks for your comments. As I argued in my review (e.g. Section 15), I would say that the HZ is *not* intrinsically tied to finding Earth-like life (whatever “Earth-like” means as I also argue against using), besides the basis on liquid water assumed in current iterations of the navigational tool.

For instance, some often assume that methanogens and other “Earth-like” staples can evolve in CO2-rich 10 bar atmospheres near the outer edge or around M-stars, which emit radiation at completely different wavelengths than our Sun. However, there is no reason to assume that at all because there is no reason to expect that evolution would follow a similar trajectory on planets with such different stellar and atmospheric characteristics. The environment of Earth is uniquely suited to support life of its own type, not some other type of life. We have some evidence of this. The lifeforms that have existed on the Earth have greatly changed over time as the planet itself has changed.

Compared to Earth, HZ definitions span a much wider range of possibilities, begetting atmospheric compositions that are very much not like the Earth. If life can arise in such habitats, there is no reason to a priori think it will be Earth-like. There is nothing Earth-like about a potentially habitable planet with a dense multi-bar CO2-CH4 atmosphere or one composed mostly of hydrogen. Evolution could very much go down a different trajectory on such planets to produce unique life suited for survival in those environments.

It is also not clear that panspermia obviates temporal HZ constraints. I see planets as systems where their potential habitability needs to meet certain conditions for life to arise. It is not just a matter of bringing in material and expecting life would pop up. Without the proper planetary conditions (right temperature range, pressure, concentration of nutrients..etc), life is impossible. You can send in all the amino acids and life building blocks in comets all you want to Venus, and I bet that life would never arise there. The planetary environment is just too inhospitable. Accordingly, Venus does not lie within the HZ either.

The limitations in the current versions of the HZ have more to do with observational considerations than with any inherent assumptions about life’s characteristics. Even the focus on surface liquid water is partly because silicon-based life would exist on planets that are quite far away from their stars, and such life would be virtually impossible to detect with present day technology.

Perhaps I should have been more specific when I used teh term “Earth-like life”. By that, I mean a carbon baased life, using liquid water as the chemical environment. I would also include the idea that there needs to be DNA and protein analogs. This is to distinguish it from really exotic life using a different atom as the backbone, different environments, e.g. Titanian hydrocarbons or gas giant atmospheres, and possibly artificial life such as machine ecosystems. Clearly some of these will exist outside of HZs where liquid water at or near the surface is a defining characteristic.

Given that we have no examples of extra-terrestrial life, we have no no empirical knowledge of how abiogenesis can occur on other worlds around different stars. However, that also implies that your suggestion about evolutionary trajectories is just speculative. We just don’t know. I also think that your assumption that Earth could not support different lifeforms is unwarranted, unless you mean non-Earth-like life as I noted above. But note, machine life could do fine on Earth, as our technological infrastructure demonstrates.

As for panspermia, I am not implying abiogenesis as you seem to suggest. I am simply suggesting that life may appear on a world in the HZ, then successfully migrate to another world that later enters the HZ, even as the genesis world eventually leaves it. If interstellar panspermia is possible, then life could evolve new forms even around short-lived hot stars. As our sun warms, Earth will become inhospitable to all but lithophillic life, but Mars may become suitable. Different arrangements of planets around other stars may make this possibility much more likely.

If we consider that technological life may have colonized the galaxy before us, then we might expect life to be in far more places than we expect. After all, SETI depends on technological life appearing elsewhere.

This is exactly why I don’t like the term “Earth-like” life. Everyone that uses it implicitly means something different. Martyn Fogg had a distinction between “biocompatible” life and “human-friendly” life. A return to clear terminology like that may be in order.

All of the HZs in my review paper assume that liquid water is essential for life. However, my point has been, that even with that restriction, life can be amazingly diverse just because there are so many other variables involved aside from just the presence of liquid water (which is just one factor).

I do not think that my suggestion about evolution is speculative (although we can’t predict exactly what life may evolve). Life does seem adapted for the environment in which it lives. As one simple example, oxygen is a poisonous gas which killed off a lot of Earth’s previous ecosystem. None of that life can thrive today in our oxygenated environment (and even then it can still be toxic to present life at high enough amounts). When conditions became right on Earth, large animals and planets appeared which were not available previously.

I had mentioned that there is no reason to think that life in a dense multi-bar CO2-CH4 atmosphere would be like Earth’s. What I mean here is not that life would not require liquid water, but that such an environment would be absolutely toxic for present humans and other lifeforms without extreme measures taken. It is worth noting that much of the HZ is unsuitable to humans (e.g. Kasting et al. 1993), which is another reason why calling life that may arise on HZ planets “Earth-like” a misnomer (but I digress). Alternatively, there would have to be some major evolutionary adjustments (perhaps even bio-engineering) for present life to cope in such different environments. But then it becomes a different species, by definition, unlike what is presently on our planet. Yes, it still uses liquid water but it is not the same species.

The pre-main-sequence HZ is especially important for M-star systems. Your example of panspermia could happen (although it is very speculative and not needed.. the real issue is if planets can migrate into the HZ at all) but there are issues. The superluminous phase for M-stars can last hundreds of millions if not up to ~2 billion years, which is much longer than gas disk survival timescales when most such migration would likely occur. Thus, such planets would have to contend with superluminous conditions and very high radiation levels during the pre-main-sequence, which may not be the best conditions for life to emerge. As I mention in my review, the HZs should be used to rank the “potential habitability” of the various planets we find. I am not claiming that a planet that was not always in the HZ is definitely not habitable, but *all* relevant factors should be considered and weighed (which makes the pre-main-sequence HZ another important tool in the box). If we have a list of planetary candidates and are trying to decide which of these are most likely to support life, such a planet should be ranked lower in importance than the planet that has always been in the HZ (all else equal). In other words, if we can only choose to further study one of the two planets (and we had not other information), I put my money on the one that was always in the HZ.

Finally, life can certainly be in many more places than we expect. But, we are focusing on life that is presently detectable. Hence, the focus on this liquid water HZ.

Of course the prospect for habitability is not solely dependent on the position of a body in a stellar HZ. We have convenient local counterexamples in Venus on the negative side and Enceladus on the positive side.

Intrinsic planetological characteristics are at least as important. Liquid water bodies are regarded as a critical element. However, Earths thin hydrosphere with its mix of dry continents and oceanic basins will remain something of a rare border case, I believe. Such a mix is very useful both in terms of optimising the chances of life’s emergence as well as it’s remote detectability. Yet, most bodies will fall outside the narrow hydrological boundary conditions which permit this. At 0.2% water by mass we live on a nearly desiccated world. Just slightly less would eliminate the hydrosphere and for lighter planets like ours prevent sustained plate tectonics. And yet less than 1% H2O would inundate the planet with a deep global ocean with corresponding problems. These include, depending on oceanic depth, dense ice phases blanketing the sea floor and excess mantle hydration which could disrupt or halt the plate tectonic process. I think the astrobiological viability of the Super Earth concept must be called into some question. Super Earths having a number of limitations not least of which is their likely invariant massive water content.

Welcome back Paul! Ramses, could you say something about the mainstream consensus on the likelihood, given a planet safely within the liquid-water portion of the HZ, that the *amount* of water present should be suitable for life? By which I mean that there is enough water, but not so much that there is no land (which would make it difficult for technological civilizations to arise). It seems I read something recently that was quite pessimistic, in that the wide range of plausible quantities of water a planet could be expected to have, included only a thin portion that we could characterize as neither too little nor too much. BUT I don’t know if that was one researcher’s pessimistic take or a disciplinary consensus. Thank you for your opinion!

I’d be wary about “disciplinary consensus”, which have been proven wrong again and again throughout history… :)

However, there is no consensus on this topic because many of us do believe that life is possible in environments that are not necessarily similar to Earth’s. My own review summarizes a lot of these viewpoints from many researchers.

Among researchers that believe life elsewhere needs to be very similar to Earth’s, some believe that other planets must also have water inventories similar to Earth’s (<0.1% of the planet's mass). This is because if water contents get much larger than this, continents can get submerged and even volcanism/plate tectonics can effectively cease. However, we have argued (Levi et al. 2017; Ramirez and Levi, 2018) that different planetary cycles would develop in place of plate tectonics on such ocean worlds, providing the ingredients needed for the emergence and maintenance of life.

I definitely enjoyed reading that review, thanks! Ocean worlds are an interesting one, nice to see there might be some ways to stabilise the climate on planets where there aren’t any exposed continents. There has been some suggestion of a universal mass-spin relationship for planets, from what I can tell this would suggest that the majority of ocean worlds would be spinning too slowly for the clathrate mechanism to work. It’d be nice to get some actual data for these planets though.

Also it’s good to see A-type stars being mentioned in terms of the possibility for habitability, always thought the lifetimes of the late-A stars were long enough for some kind of life to emerge.

A laudable effort by Dr. Ramirez to refine and clarify criteria for exoplanetary habitable zones.

The question that comes to mind is whether other thinkers in the field such as Fred Hoyle (The Black Cloud) and Robert Forward (Dragon’s Egg) were out of left field or within the realm of rationality. If the latter, we could extend habitability to black clouds and the surface of neutron stars, in which case the Universe may indeed be a stranger place than we imagine.

Hi Dr. Ramirez=
There are just too many questions I want to discuss with you in person.

1) The proposed climate stabilizing mechanisms on CH4- or H2-rich temperate planets are thought to be quite biological/gaian in nature, so a prerequisite is the presence of life in large quantities (in order to modulate atmospheric composition). We should be aware, however, of the risk of a chicken-and-egg problem if we call for a process that can happen only within the condition of widespread life.
In contrast, a CO2 atmosphere like Earth can be regulated by the inorganic carbon cycle as you said.
I propose that CH4- or H2-CO2 temperate planets can still be regulated by carbonate-silicate cycle. If the weathering rate has very strong dependence on surface temperature, it should stabilize the temperature by only changing pCO2 without making a change to CH4 or H2, because altering the ratio of CO2 and other greenhouse gases concentration can possibly achieve a new equilibrium. I wonder if this process can be tested in a sophisticated weathering-climate coupled model. I haven’t seen any research done on this topic, it should be an interesting one.

2) Another problem is mantle redox evolution. CH4 and H2 are highly reduced gases resulted from the outgassing of a highly reduced mantle. On an Earth-sized or a larger rocky planet, mantle should be oxidized rapidly (less several hundred million years perhaps) after its formation. The outgassed C and H should be dominantly in the forms of CO2 and H2O, and CH4 and H2 are just minor components. CH4 might be produced by serpentinization, but it would be hard to compensate the rapid escape if no biotic source is involved. If we assume there is a biotic source for CH4 production, this assumption would just create another chicken-and-egg problem.
Whether enough H2 would be generated from an oxidized mantle is also questionable. Massive planets might be able to retain “some” primordial hydrogen envelop from accretion. However, a super-Earth formed and accreted before disk dissipation and perfectly retained only so little gas is highly unlikely (though odds exist), because the envelop mass fraction usually accounts for at least 0.001 of the planet mass after periods of intense stellar activity (Stökl et al., 2016).

3) Early water escape on M-dwarf main-sequence habitable-zone planets is overly pessimistic. Calculations showing a lost of tens of oceans of water like Luger & Barnes (2015) overestimated ε and XUV cross-section in the formula (Bolmont et al., 2017). Using improved energy-limited escape model, new water escape estimates are significantly lower than previous ones. No more than 3 oceans are obtained for low-mass stars (Bolmont et al., 2017; Ribas et al., 2017; Bourrier et al., 2017).
Additionally, early runaway greenhouse would only desiccate the surface portion of the water, and not all the water is outgassed onto the surface during early evolution. As a planet forms, some water is naturally incorporated into crystalizing mantle minerals, and this process enrich the upper mantle up to 1 wt% water, corresponding to ~10 oceans of water (Tikoo & Elkins-Tanton, 2017). It can be constantly released back on surface as volcanic and tectonic outgassing during the long history of planetary evolution. Though in this case, the PMS lost amount would be determined by the duration of magma ocean, it could last longer on M-dwarf planets.
Venus Express has captured young felsic continent-like crust on Venus (Smrekar et al., 2018). A huge volume of felsic magma could only be generated from partial melting of basalt in the presence of water, indicating Venus interior still holds some water.

4) CO2 condensation limit in Turbet et al (2017) should be valued. Earth experienced low latitude glaciations at least three times in the past. Planets receiving less stellar flux are even more susceptible to widespread glaciations. If Earth-like planets receiving less than 820 W/m2 is ever glaciated, polar CO2 condensation would offset volcanic outgassing and global glaciation becomes permanent. This study should expand more to include other types of stars, especially K and F. To me, the conservative habitable zone of solar-like stars is not conservative enough.

Thank you for your comments! I am glad you liked the article. You say a lot here but I will touch on some key points..

1) I also agree that the CO2-CH4 and CO2-H2 mechanisms can be sustained by the carbonate-silicate cycle. In fact, this is what is assumed in these papers. You are right that it would be a good idea for this to be shown explicitly, however.

2) I agree that maintaining a H2-rich atmosphere through volcanic outgassing on a larger terrestrial planet may seem challenging (as I mention in the review), but I do not know (without observations) if it is impossible, especially on a planet with a very different redox budget than the Earth. The arguments that myself and others (e.g Pierrehumbert and Gaidos, 2011) have invoked other things that may help like potent magnetic fields and higher volcanic outgassing rates.

3) The tens to hundreds of Earth oceans I give in my review paper are upper limits on water losses that I deem to still be quit accurate. I don’t think the papers by Bolmont, Bourier, Ribas really change the picture here, even with assuming a lower heating efficiency (eta). How to treat the XUV flux evolution before flux saturation is quite poorly-understood and very different answers can be obtained by doing this calculation different ways. Moreover, stellar rotation rate *greatly* impacts volatile losses. The volatile loss estimates can vary by orders of magnitude. Even with the low-ball numbers given by Bourier et al. (2017), their Tables 6 – 7 are consistent with maximum water losses in the tens to hundreds of Earth oceans. Note that pre-main-sequence lifetime for an M8 (like TRAPPIST-1 approximately is) can be over ~2 billion years. I have done this calculation myself and I can easily show that tens to hundreds of Earth oceans of water can be lost during the pre-MS (particularly for the late M-stars), even assuming lower heating efficiencies.

I also agree that a runaway greenhouse need not completely desiccate a planet’s interior. The repeated resurfacing events in Venus’ history might be evidence of this. However, I would still suspect that water on the surface of such a planet would not be easily maintained even if it is periodically belched. From a habitability standpoint, such a planet may not be so attractive (look at Venus). Interesting problem though.

4) Kasting et al. (1993) had originally calculated a CO2 condensation limit for F – M stars. Something like that may indeed be the true outer edge of the classical HZ. However, this is one reason secondary greenhouse gases, like CH4 and H2, are so important. As I also mention in the review, the additional heating from these gases would stave off atmospheric collapse to higher Co2 pressures, pushing the outer edge outward.

Have you done calculations on how long and how much cometary material would be around M8 dwarfs as in the Trappist 1 system after the first 2 billion years? Since these are minature solar system, material from the icy comets could replenish H2O in a much faster time period because of the larger areas and faster periods that these planets travel around their sun.

Hi Dr. Ramirez,
Thanks for replying and presenting your views on these questions.

1) I am looking forward to the future climate stabilizing mechanisms study! This newborn field is fascinating.

2) Since we do not thoroughly understand planetary evolution, reduced mantle on an Earth-size or super-Earth planet remains a possibility. I would like to see if exogeophysicists can discover some hidden mechanisms that would argue for it.

3) I see your paper “The habitable zones of pre-main-sequence stars” gives a number between hundreds to thousands of oceans that could be lost during PMS runaway greenhouse phase (RGP). As you have explained, this discrepancy is due to choosing different evolutionary tracks of XUV flux of low-mass stars, not model parameters.
In your paper, you generalized all types of M-dwarfs (M1 to M9) XUV flux by using a single empirically derived age-based scaling law that is apparently unrelated to bolometric luminosity. For example, it predicts all M-dwarfs at age of 50 Myr have XUV flux as high as 10^29 ergs/s.
In Ribas et al paper, they assume the XUV flux is a function of bolometric luminosity prior to saturation phase, predicting orders of magnitude lower PMS XUV flux.
Which one do you think is a better method to evaluate early RGP?

On a planet with desiccated surface, CO2 would likely be the dominant atmospheric gas after RGP and magma ocean, an extremely thick CO2 atmosphere I imagine. Your study (doi.org/10.1089/ast.2014.1153) has shown further increase in pCO2 does not trigger RGP but moist greenhouse phase, so volcanic CO2 outgassing followed RCP should not matter. Assuming the outgassed H2o rate is similar to current Earth 10^14 mol / yr (Catling & Kasting, 2017), hence, it is possible to image that magma outgassing can naturally restore half of Earth ocean within 0.5 Gyr, and there is no water sink because plate tectonics require a large quantity of water on the surface first, and before that, carbonate-silicate cycle should have already started to operate and drawdown pCO2.

2) Regarding mantle evolution, we definitely do not understand it well. Some of my colleagues do not even believe the mantle redux mechanism of Wade and Wood (2005) and I am not sure that I do either. My colleague (Amit Levi) works on some of this exogeophysics but I think we also need more old-fashioned Earth geophysical lab studies that can assess these strange scenarios.

3) The atmospheric losses in my 2014 paper used the XUV/EUV flux parameterization of Lammer et al. (2009), which predicts much higher fluxes than the parameterizations that have followed suit, and so water loss estimates for that can be ~an order of magnitude higher than in subsequent papers. When I worked on this initially, the idea was just to obtain a rough estimate of the water losses and show that it is worse for M-stars than other star types (which I think it sufficiently showed). In contrast, Luger and Barnes (2015) do a more careful treatment of the EUV flux parameterization, including dealing with the problem of flux saturation.

As you mention, the Lammer et al. (2009) parameterization does not distinguish between different temperature stars within a class (e.g. G, M..etc), so there is definitely error there. Even for the Sun, it predicts somewhat higher water losses than the generally-accepted Ribas (2005) parameterization. In light of all of this, I now favor devising appropriate flux parameterizations for individual stars.

Thank you for taking the time to answer my questions!
The last one is the discrepancy between Luger & Barnes (2015) and Ribas et al (2017).
I looked over their stellar evolution models at the same stellar mass (Proxima Centauri mass) and found no significant divergence in XUV flux and luminosity evolution during PMS and saturation phase. What could have contributed to cause the huge difference between Luger & Barnes estimation of over 10 oceans and Ribas et al estimation of 2 oceans in the HZ of Proxima-like stars?
In fact, Luger & Barnes assumes an even shorter early RGP than Ribas et al did, and log(Lxuv/Lbol) = -3 during PMS and saturation phase in Luger & Barnes is even smaller than the -2.78 that Ribas et al has assumed in their early ending saturation model.
Are the differences in their results caused by heating efficiency, different treatments of oxygen escape, first 10 Myr protection of protoplanetary disk (Ribas et al assumes no loss during first 10 myr due to disk protection), or something else?
I thought the heating efficiency decreasing by a factor of 3—from 0.3 to 0.1—is quite important in the case of small loss maybe?

As Tian (2015) showed, Luger and Barnes (2015) did not allow the H and O ratio to evolve with time, keeping it constant between the 2 species. This could also be behind some of the discrepancies, especially since many of the later studies are allowing H and O to evolve. Also, as you mentioned, different heating efficiency values can also cause large changes.

We pretty much all agree that a planet has to be in the life belt of a star for life to evolve and survive, but that is not enough. There are other necessities like a magnetic field to stop solar wind stripping and hold an atmosphere over time. I have argued that a planet has to have a Moon to have a strong enough magnetic field but not be too large a planet. One has to do the necessary application of scientific principles which limit the type of environment that life can evolve. I still think the Earth moon example is the best one which is a single example, but since scientific principles are based on physics which apply everywhere in the entire universe. Consequently, a single example is all we need since the principles of physics and science ARE a priori as first principles and are discovered, but not invented so they were always here before us.

The problem with the pre main sequence atmosphere is that it does not hang around very long like CH4 in the early atmosphere. There is a T tauri phase of strong solar wind of stars which blows away the original gases of the star system.

There can be no such thing as silicon based life because there cannot be enzymes with silicon. The enzymes have to have both left and right handed molecules. Consequently, without enzymes there cannot be any bacteria to decompose life through decay and silicon based life would not be hardy but fail. Source: Could silicon be the basis for alien life forms, just as carbon is on Earth? Scientific American. https://www.scientificamerican.com/article/could-silicon-be-the-basi/

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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